The decreasing availability of high grade copper ores necessitates utilization of lower grade ores, which often contain substantial amounts of iron. A processing method which promises to become important for recovery of copper from such ores comprises acid-leaching, liquid/liquid extraction and acid stripping. However, in order for this method to be economic for copper ores which yield acid leaches of high iron content, it is necessary to employ extractants which are highly selective for copper over iron.
U.S. Pat. No. 4,020,081 discloses that 2-(2-hydroxyphenyl)-benzoxazoles of the formula ##STR2## wherein R is H or alkyl of 1 to 20 carbons, R.sup.1 is H, Cl or alkyl of 1 to 20 carbon atoms, X is H or Cl and at least one of R and R.sup.1 must contain 6 or more carbons, have utility for extraction of copper (and other metal values) from ammoniacal aqueous solutions having pH's of from about 7 to about 10.5.
The patent infers that the disclosed compounds will extract metal values from other types of aqueous solutions but is silent as to specifics in this regard. Leaches having pH's in the latter range will not contain significant amounts of iron and selectivity for copper over iron is not required of the disclosed compounds.
In the course of a search for compounds which would selectively extract copper from unneutralized acid leaches, several benzoxazoles of the preceding formula were independently conceived, made and tested by the present inventors. The results were disappointing; the extent and/or rate of copper extraction (or stripping) was too low or poor selectivity for copper over iron was found. However, the compounds were otherwise well suited for the intended purpose and the possibility of improving their performance by introducing various types of substituents in the prototype 2-(2-hydroxyphenyl) benzoxazole molecule was considered.
To be suitable for practical use as an extractant, a candidate compound must simultaneously satisfy a minimum of eight different requirements (which are familiar to those skilled in the art; see column 1 of the '081 patent). This poses a problem, in that a change in composition intended to make the compound more suitable in one respect may also make it less suitable in another respect. Thus, for example, a modification which increases the ability of the compound to take up copper ions at a pH of about 2 may adversely effect solubility properties and/or unduly increase acid requirements for stripping.